The Heart Sūtra is a popular sutra in Mahāyāna Buddhism. Its Sanskrit title, Prajñāpāramitāhṛdaya, can be translated as "The Heart of the Perfection of Wisdom".

The sutra famously states, "Form is empty" (śūnyatā). It is a condensed exposé on the Buddhist Mahayana teaching of the Two Truths doctrine, which says that ultimately all phenomena are sunyata, empty of an unchanging essence. This emptiness is a 'characteristic' of all phenomena, and not a transcendent reality, but also "empty" of an essence of its own. Specifically, it is a response to Sarvastivada teachings that "phenomena" or its constituents are real. (source: Wikipedia)